In the processing of a semiconductor wafer to form integrated circuits, charged atoms or molecules are directly introduced into the wafer in a process called ion implantation. Ion implantation normally causes damage to the lattice structure of the wafer, and to remove the damage, the wafer is normally annealed at an elevated temperature, typically 600xc2x0 C. to 1100xc2x0 C. Prior to annealing, material properties at the surface of the wafer may be measured, specifically by using the damage caused by ion implantation.
For example, U.S. Pat. No. 4,579,463 granted to Rosencwaig et al. (that is incorporated herein by reference in its entirety) describes a method for measuring a change in reflectance caused by a periodic change in temperature of a wafer""s surface (see column 1, lines 7-16). Specifically, the method uses xe2x80x9cthermal waves [that] are created by generating a periodic localized heating at a spot on the surface of a samplexe2x80x9d (column 3, lines 54-56) with xe2x80x9ca radiation probe beam . . . directed on a portion of the periodically heated area on the sample surface,xe2x80x9d and the method xe2x80x9cmeasures] the intensity variations of the reflected radiation probe beam resulting from the periodic heatingxe2x80x9d (column 3, lines 52-66).
As another example, U.S. Pat. No. 4,854,710 to Opsal et al. (also incorporated herein by reference in its entirety) describes a method wherein xe2x80x9cthe density variations of a diffusing electron-hole plasma are monitored to yield information about features in a semiconductorxe2x80x9d (column 1, lines 61-63). Specifically, Opsal et al. state that xe2x80x9cchanges in the index of refraction, due to the variations in plasma density, can be detected by reflecting a probe beam off the surface of the sample within the area which has been excitedxe2x80x9d (column 2, lines 23-31) as described in xe2x80x9cPicosecond Ellipsometry of Transient Electron-Hole Plasmas in Germanium,xe2x80x9d by D. H. Auston et al., Physical Review Letters, Vol. 32, No. 20, May 20, 1974.
Opsal et al. further state (in column 5, lines 25-31 of U.S. Pat. No. 4,854,710): xe2x80x9cThe radiation probe will undergo changes in both intensity and phase. In the preferred embodiment, the changes in intensity, caused by changes in reflectivity of the sample, are monitored using a photodetector. It is possible to detect changes in phase through interferometric techniques or by monitoring the periodic angular deflections of the probe beam.xe2x80x9d
A brochure entitled xe2x80x9cTP-500: The next generation ion implant monitorxe2x80x9d dated April, 1996 published by Therma-Wave, Inc., 1250 Reliance Way, Fremont, Calif. 94539, describes a measurement device TP-500 that requires xe2x80x9cno post-implant processingxe2x80x9d (column 1, lines 6-7, page 2) and that xe2x80x9cmeasures lattice damagexe2x80x9d (column 2, line 32, page 2). The TP-500 includes xe2x80x9c[t]wo low-power lasers [that] provide a modulated reflectance signal that measures the subsurface damage to the silicon lattice created by implantation. As the dose increases, so does the damage and the strength of the TW signal. This non-contact technique has no harmful effect on production wafersxe2x80x9d (columns 1 and 2 on page 2). According to the brochure, TP-500 can also be used after annealing, specifically to xe2x80x9coptimize . . . system for annealing uniformity and assure good repeatabilityxe2x80x9d (see bottom of column 2, on page 4).
U.S. Pat. No. 5,978,074 discloses focusing a probe beam onto a sample surface withn an area periodically excited by an intensity modulated generation beam. The power of the reflected probe beam is measured by a photodetector, and the modulated optical reflectivity of the sample is derived. Measurements are taken at a plurality of pump beam modulation frequencies. In addition, measurements are taken as the lateral separation between the pump and probe beam spots on the sample surface is varied.
At column 3, lines 23-30, U.S. Pat. No. 5,978,074 states that xe2x80x9cin the prior art, the modulation range was typically in the 100 KHz to 1 MHz range. Some experiments utilized modulation frequency as high as 10 MHz. In the subject device, it has been found useful to take measurements with modulation frequencies up to 100 MHz range. At these high frequencies, the thermal wavelengths are very short, enabling information to be obtained for thin metal layers on a sample, on the order of 100 angstroms.xe2x80x9d
At column 8, lines 22-27, U.S. Pat. No. 5,978,074 further states xe2x80x9cOnce all measurements at various spacings and modulation frequencies have been taken and stored, the processor will attempt to characterize the sample. Various types of modeling algorithms can ve used depending on the complexity of the sample. Optimization routines which use iterative processes such as least square fitting routines are typically employed.xe2x80x9d
Abruptness of a junction in a semiconductor wafer can be measured by Secondary Ion Mass Spectrometry (SIMS) in which a wafer is milled away using an ion beam, and the removed material is analyzed. Alternatively, abruptness of a junction can also be determined from electrical characteristics of transistors or other test structures on a completely fabricated integrated circuit (IC) device, having contacts.
An apparatus and method in accordance with the invention use a computer to determine a coefficient in a function that relates measurements from a wafer, to values of a parameter used in making the measurements, and use the coefficient to evaluate the wafer (e.g. to accept or reject the wafer for further processing), and/or to control fabrication of another wafer. For example, the apparatus and method may use a second order coefficient of a power series function to control operation of a wafer processing unit (that may include, e.g. an ion implanter), or a heat treatment unit (such as a rapid thermal annealer).
In one embodiment, two beams are used to make measurements that are input to the computer: one beam (called xe2x80x9cgeneration beamxe2x80x9d) of photons having an intensity modulated at a frequency sufficiently low to avoid creation of a wave of charge carriers in the wafer, and another beam (called xe2x80x9cprobe beamxe2x80x9d) used to measure the concentration of charge carriers created by the generation beam. Specifically, a photosensitive element located in a path of a portion of the probe beam that is reflected by the charge carriers (and that is therefore also modulated) generates an electrical signal indicative of the concentration. Such measurements are related by the above-described function to the above-described parameter (which may be, for example, power of the generation beam, power of the probe beam, or spot size).